Alkane dehydrogenation can be a valuable process in which saturated hydrocarbons are converted to alkenes (olefins) and hydrogen (H2). Examples of alkane dehydrogenation processes can include conversion of branched or unbranched C2 to C19 alkanes to the corresponding C2 to C19 alkenes, e.g., conversion of ethane to ethylene, propane to propylene, isobutane to isobutylene, and ethylbenzene to styrene. Alkane dehydrogenation reactions can be performed with heterogeneous catalysts.
Alkane dehydrogenation reactions can be performed with metal-based heterogeneous catalysts. The catalysts can contain metal oxides and/or elemental metal surfaces (active surfaces). The catalysts can be based on platinum (Pt) and other Group VIII metals. Other catalysts can include chromium (Cr) and/or chromium oxide. Additional heterogeneous catalysts for alkane dehydrogenation have also been reported. An alkane feed stream can be passed over or through the heterogeneous catalyst to produce a product stream containing an alkene and hydrogen. The heterogeneous catalyst can be positioned in a catalyst tube, catalyst bed, or catalyst-filled plate. Alkane dehydrogenation reactions are endothermic and can be conducted at a high temperature. For example, the alkane feed stream can be preheated to a temperature in the range of about 550° C. to about 700° C. and the reaction conducted within the same temperature range. At such high temperatures, side reactions can occur and coke (carbon residue) can form. Carbon deposition on the surface of heterogeneous catalysts can reduce catalytic activity. Thus, over time, conversion and efficiency of alkane dehydrogenation can decline if catalysts are not regenerated or replaced.
Choice of catalyst can influence the optimal parameters for alkane dehydrogenation. Certain Cr-based catalysts can be highly active and achieve high conversion of alkanes to alkenes but can be deactivated relatively quickly, which can require relatively frequent regeneration, e.g., after about 5 to 20 minutes of operation. Certain Pt-based catalysts can be less active than Cr-based catalysts but can have improved resistance to deactivation and can require relatively infrequent regeneration, e.g., after about 6 to 12 hours of operation.
Accordingly, alkane dehydrogenation catalysts can require periodic regeneration. Catalysts can be regenerated by oxidation (burning) of carbon residues and, if necessary, reduction of the catalyst to restore catalytic activity. For example, regeneration of Pt- or Cr-based alkane dehydrogenation catalysts can involve (1) purging with steam, (2) exposure to a regeneration mixture stream containing steam and oxygen (which can oxidize carbon residues), (3) reduction, and (4) evacuation. Catalyst regeneration can be conducted at high temperature and can in many instances be conducted within the same temperature range at which the alkane dehydrogenation reaction is conducted (e.g., about 550° C. to about 700° C.).
Certain techniques for alkane dehydrogenation can involve separate heating of (1) the alkane feed, (2) the dehydrogenation reaction catalyst tube(s), bed(s), or plate(s), and (3) the regeneration mixture feed. Separate heating of each component can be expensive due to high energy consumption and can also require complex reactor systems. Thus there remains a need for in the art for techniques for alkane dehydrogenation with reduced energy consumption and simplified operation.